Re: [PATCH v1 02/11] VFS permit cross device vfs_copy_file_range

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On Fri, Oct 19, 2018 at 6:54 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>
> On Fri, Oct 19, 2018 at 6:30 PM Olga Kornievskaia
> <olga.kornievskaia@xxxxxxxxx> wrote:
> >
> > From: Olga Kornievskaia <kolga@xxxxxxxxxx>
> >
> > Allow copy_file_range to copy between different superblocks but only
> > of the same file system types. This feature was of interest to CIFS
> > as well as NFS.
> >
> > This feature is needed by NFSv4.2 to perform file copy operation on
> > the same server or file copy between different NFSv4.2 servers.
> >
> > If a file system's fileoperations copy_file_range operation prohibits
> > cross-device copies, fall back to do_splice_direct. This would be
> > needed for the NFS (destination) server side implementation of the
> > file copy and currently for CIFS.
> >
> > Besides NFS, there is only 1 implementor of the copy_file_range FS
> > operation -- CIFS. CIFS assumes incoming file descriptors are both
> > CIFS but it will check if they are coming from different servers and
> > return error code to fall back to do_splice_direct.
> >
> > NFS will allow for copies between different NFS servers.
> >
> > Adding to the vfs.txt documentation to explicitly warn about allowing
> > for different superblocks of the same file type to be passed into the
> > copy_file_range for the future users of copy_file_range method.
> >
> > Signed-off-by: Olga Kornievskaia <kolga@xxxxxxxxxx>
> > ---
> >  Documentation/filesystems/vfs.txt |  4 +++-
> >  fs/read_write.c                   | 13 ++++++-------
> >  2 files changed, 9 insertions(+), 8 deletions(-)
> >
> > diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
> > index a6c6a8a..5e520de 100644
> > --- a/Documentation/filesystems/vfs.txt
> > +++ b/Documentation/filesystems/vfs.txt
> > @@ -958,7 +958,9 @@ otherwise noted.
> >
> >    fallocate: called by the VFS to preallocate blocks or punch a hole.
> >
> > -  copy_file_range: called by the copy_file_range(2) system call.
> > +  copy_file_range: called by copy_file_range(2) system call. This method
> > +                  works on two file descriptors that might reside on
> > +                  different superblocks of the same type of file system.
> >
> >    clone_file_range: called by the ioctl(2) system call for FICLONERANGE and
> >         FICLONE commands.
> > diff --git a/fs/read_write.c b/fs/read_write.c
> > index c60790f..474e740 100644
> > --- a/fs/read_write.c
> > +++ b/fs/read_write.c
> > @@ -1578,10 +1578,6 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
> >             (file_out->f_flags & O_APPEND))
> >                 return -EBADF;
> >
> > -       /* this could be relaxed once a method supports cross-fs copies */
> > -       if (inode_in->i_sb != inode_out->i_sb)
> > -               return -EXDEV;
> > -
>
> You need to hoist this limitation to  clone_file_range() syscall, because
> you are not allowed to change user facing behavior.
> Maybe you can later add a uapi flag for copy_file_range() to explicitly allow
> for cross-sb copy?
> Maybe you can add the flag now for internal use  - only nfsv4 will pass that
> flag to the vfs helper and system call will verify that flags == 0.
>

Sorry, you meant to allow cross sb copy from client, so you do want
to change user facing behavior.
For that you need to declare a flag to opt in for the new behavior
and document it.
Please keep linux-api CC-ed when posting the flag.
Referencing a draft for man page is recommended.

Thanks,
Amir.



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